Dexlansoprazole, namely (R)-2-[[[3-methyl-4(2,2,2-trifluoroethoxy)-2-piridyl]methyl]sulfinyl]benzimidazole, having formula (I), is the (R) enantiomer of Lansoprazole, which is a protonic pump inhibitor of the gastric parietal cells and therefore useful in therapy in the treatment of several disorders of the gastrointestinal tract.

WO 96/02535 and EP 1277752 disclose the synthesis of Dexlansoprazole by stereoselective oxidation, according to the Kagan-Modena method, of the prochiral sulphur of formula (A) in toluene. Anyway, the stereoselective oxidation of the prochiral sulphur of formula (A), even if deeply studied, is a complex reaction when carried out on industrial scale and, generally, brings to the formation of a mixture of products comprising the sulphoxide of formula (I) having the desired stereochemistry, the enantiomer thereof, variable quantities of the staring sulphur of formula (A), and the superoxidation sulphur having formula (B).

The purification of this mixture is often complicated and requires several crystallization steps to obtain a solid product sufficiently pure from a chemical and stereochemical point of view. In particular removal of the compound (B) is a known problem in the production of chiral sulfoxides and is a real issue in the preparation of chiral sulfoxides on industrial scale.